Medicine & USMLE

Homocysteine Metabolism

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Biochemical Pathways
  1. Glycolysis
  2. Citric Acid Cycle (TCA Cycle)
  3. Electron Transport Chain (ETC)
  4. Cori Cycle
  5. De Novo Purine Synthesis
  6. De Novo Pyrimidine Synthesis
  7. Purine Salvage
  8. Purine Excretion
  9. Ethanol Metabolism
  10. Pyruvate Metabolism
  11. HMP Shunt (Pentose Phosphate Pathway)
  12. Galactose Metabolism
  13. Sorbitol (Polyol) Pathway
  14. Urea Cycle
  15. Alanine (Cahill) Cycle
  16. Catecholamine Synthesis & Breakdown
  17. Homocysteine Metabolism
  18. Fatty Acid Synthesis (Citrate Shuttle)
  19. Fatty Acid Breakdown (Carnitine Shuttle)
  20. Propionic Acid Pathway
  21. Fructose Metabolism
  22. Regulation by Fructose-2,6-Bisphosphate (F-2,6-BP)
  23. Glycogenesis
  24. Glycogenolysis

Homocysteine can be metabolized down two different pathways: (1) methylation to methionine, (2) or transsulfuration to cystathionine with the eventual formation of cysteine.

Production of methionine from homocysteine is catalyzed by methionine synthase, which requires the Vitamin B9 (Folate)-derived 5-MTHF and Vitamin B12 (Cobalamin). Therefore, decreases in either folate or cobalamin can lead to increases in homocysteine (see Vitamins B9 and B12 Deficiencies). Notably, 5-MTHF is regenerated by MTHFR, and defects in MTHFR can lead to excess homocysteine and homocystinuria (see Homocystinuria - coming soon).


Production of cystathionine and eventually cysteine occurs through a different set of reactions. Namely, cystathionine synthase combines homocysteine and serine to produce cystathionine, using Vitamin B6 (Pyridoxine) as a cofactor. The cystathionine is then cleaved to produce cysteine.

Find this Homocysteine Metabolism mnemonic and more Biochemical Pathways mnemonics among Pixorize's visual mnemonics for the USMLE Step 1 and NBME Shelf Exams.